That violation and the Lung Association's grades -- which cover the period from 2005 to 2007 -- are not an indication that Seattle's air became dirtier. Rather, they reflect stricter smog standards that the Environmental Protection Agency imposed last year.

"Many counties got an F for the first time," Janice Nolen, assistant vice-president of national policy and advocacy for the Lung Association, said Tuesday.

"That does not necessarily mean the air is getting dirtier. It means we've had unhealthy air all along."

The agency's annual "State of the Air" report analyzed data for ozone and fine particle pollution -- the most common types of air pollution -- and found that six out of 10 Americans were living in areas where pollution endangers lives.

"This should be a wake-up call. We know that air pollution is a major threat to human health," said Stephen Nolan, chairman of the Lung Association's national board.

The pollutants can damage the developing lungs of children; exacerbate asthma, bronchitis and other lung conditions; and increase the risk for heart attacks, stroke and premature death.

The report found that Los Angeles, the country's top ozone-polluted city, had made consistent strides toward cleaner air, while Las Vegas and the Dallas-Ft. Worth region had higher ozone levels than a decade ago.

Bakersfield, Calif. was designated the country's top city for short-term spikes in particle pollution, a mix of microscopic soot, diesel exhaust and chemicals. Pittsburgh, PA held the No. 1 spot for most year-round particle pollution.

Fargo, N.D., was named the cleanest city in all three pollution categories. Other top clean cities were Honolulu, Tucson, and Billings.

The report found that King County had eight days in a three-year period in which ozone levels were unhealthy for children, older people, and people with lung diseases. The county also had three days of ozone levels that were unhealthy for everyone.

The findings echoed last year's fall into federal noncompliance, a notable event for an area that prides itself on green causes.

Local officials said the report was based on older data and didn't take into account the progress in reducing diesel emissions and replacing old wood-burning stoves that have improved air quality.

"The report doesn't tell the full story," said Al Brady, chairman of the local Lung Association board. "However, it does acknowledge our concerns about the quality of the air we breathe."

The Puget Sound Clean Air Agency said it has retrofitted more than 2,500 school buses in four counties as part of a program to reduce diesel emissions in transportation fleets and water vessels. Ozone, the primary ingredient in smog, forms when emissions mix with heat and sunlight.

The report also gave Pierce and Snohomish counties an "F" for short-term fine particle pollution. Officials said a wood-stove replacement program is expected to reduce the pollutants in those areas by more than 26,000 pounds a year.

"While the air continues to get cleaner every year in the region, we do know we have more work to do to have clean and healthy air on all days," said Dennis McLerran, executive director of the Puget Sound Clean Air Agency.

"With new health-based air quality standards, the bar has been set higher for us."